1. Field of the Invention
The invention relates to microbiological methods of heavy-metal oxide or oxyanion removal from aqueous media by Rhodobacter sphaeroides. Several subgenera of Rhodobacter and related species efficiently reduce the metal oxides and oxyanions of selenium, tellurium and rhodium to the free metal which is readily isolated from the cytoplasmic membrane. These microorganisms exhibit resistance to a wide variety of oxides and oxyanions making bioremediation of selected heavy-metal oxides and oxyanions feasible, even in the presence of other oxides and/or oxyanions including those of vanadium, iodine, silicon, molybdenum, tin, tungsten, antimony and arsenic.
2. Description of Related Art
A major environmental problem exists in dealing with toxic metal compounds found ubiquitously dispersed in groundwater, lakes, plant effluents, and aqueous waste. Generally these toxic compounds are heavy metal oxides or oxyanions exemplified by the tellurite, arsenate and periodate classes of rare-earth oxyanlons and oxides. A particularly obnoxious group of contaminants identified as a threat to western United States water supplies includes the oxyanions of selenium frequently found in agricultural wastewaters (Sylvester, 1988).
Potential and actual health problems also arise due to toxic effects of many oxidized heavy metals. Exposure to tellurium compounds is hazardous to workers in the film and rubber industries, as well in battery manufacture. When accumulated in the human body, many of these elements have detrimental mental and physical effects (Schroeder et al., 1967).
Bioremediation has been explored as a method of detoxification of toxic compounds found in water. Proposed methods generally take advantage of microbiological resistance to such compounds. The basis of resistance may be metabolic breakdown or concentration of the material within the microorganism. It is known, for example, that some species of Gram-positive bacteria, such as Corynebacterium diphtheriae, Streptococcus faecalis and most strains of Staphylococcus aureus are naturally resistant to tellurite and will often concentrate metallic tellurium inside the inner membrane (Walter and Taylor, 1989). Resistance determinants to tellurite have been identified and isolated in Escherichia coli (Walter and Taylor, 1989). However, resistance to tellurite is not a common property of bacteria and examples of naturally-occurring resistant strains are rare (Chiong et al., 1988). Often times such resistance is to only low or moderate levels of these compounds, e.g. .ltoreq.100 .mu.g/ml.
A method for accelerating recovery of selenium from aqueous streams is based on bioreduction of Se(VI) to Se(IV) with strains of the soil bacterium, Clostridium. A rapid exchange reaction between selenous acid and pyrite is used to remove the selenium from solution. However, to remove selenium, further processing is required, e.g., generation of hydrogen selenide and subsequent oxidization to the free metal (Khalafalla, 1990). Clostridium species have also been utilized in a process for reducing waste-containing radionuclides or toxic metals, but the process requires obligate anaerobic conditions at elevated temperatures (Francis and Gillow, 1991).
In addition to bioremediation, microorganisms are thought to have practical value in possible reclamation of metals from such sources as low grade ores, or in recovery processing. However, while a few bacterial species have resistance to one or more metal cations under some conditions, resistance may be based on accumulation rather than a metabolic reaction. Few microorganisms have been identified that reduce metal cations to the free metal (Summers and Silver, 1978). Moreover, resistance may not be to whole classes of such compounds, but to only a few.